Stainless steel enclosures: gauge, finish, welds and care

Gauge and thickness

Sheet gauge numbers are not a unit. A stockist's gauge chart says it plainly: "Gauges are neither standard nor metric", and the same gauge number means a different thickness in different metals. Stainless sheet is slightly thicker than carbon steel at the same gauge:

SSINA's architectural guide lists the same nominal stainless values. Actual thickness varies within mill tolerances, so a drawing should state the thickness in millimetres or inches. Thicker sheet adds stiffness and weight: 304 is 7.90 g/cm³, so each square metre of 1.5 mm sheet weighs about 11.9 kg (our arithmetic). Our wall-mount enclosures use 1.2, 1.5 or 2.0 mm sheet (about 18, 16 and 14 ga) depending on size.

Surface finish: No. 4 vs 2B

ASTM A480 and EN 10088 provide the usual finish definitions. The ones you meet on enclosures:

• 2B: "a smooth, moderately reflective cold-rolled annealed and pickled or descaled finish", given a final light pass on polished rolls.
• 2D: the same process without the polished-roll pass; smooth and non-reflective.
• BA: bright annealed, a mirror-like cold-rolled finish.
• No. 4: a "general purpose bright polished finish", linearly textured, typically obtained with 120–150 mesh abrasive after coarser grinding. This is the brushed finish of our wall-mount enclosures.
• No. 8: the most reflective finish commonly produced on sheet.

Published roughness ranges overlap and do not all agree. ASSDA gives 2B as 0.1 to 1 µm Ra and No. 4 as typically about 0.2 to 0.6 µm; BSSA gives 0.1–0.5 µm for 2B and 0.2–1.5 µm for brushed finishes in its class. The direction is consistent: smoother surfaces resist corrosion better and are easier to keep clean. BSSA warns that coarse polishes above about 1 µm transverse Ra "will exhibit deep grooves where chloride ions can accumulate", and that a description such as "satin polish" does not guarantee a smooth finish. If corrosion or hygiene matters, put a maximum Ra on the drawing. For food contact surfaces, EHEDG guidance as quoted by Euro Inox is 0.8 µm Ra or better; an enclosure is not a product-contact surface, but the number is a useful reference for splash zones.

Welded seams and heat tint

Two weld details decide how a stainless enclosure ages in a wet or corrosive area.

Continuous seams. Crevices collect chloride salts, pollutants and moisture, and the low-oxygen conditions inside a crevice let chlorides attack quickly. ASSDA's food-industry code of practice states that lap joints and intermittent fillet welds "shall be avoided because of crevice and contamination issues", and its coastal guidance notes that intermittent welds trap water. For a washdown or 4X-type enclosure, ask for continuously welded corners and seams, ground smooth where they face outward.

Heat tint removal. The discolouration beside a weld is an oxide layer that has drawn chromium from the metal just below it. Euro Inox: "Visible welding heat tint on stainless steel surfaces reduces the available corrosion resistance of the surface. It is good practice to remove all visible heat tinting." Pickling (acid pastes or gels, spray or immersion) removes it; mechanical cleaning followed by pickling or passivation is the usual sequence, and BSSA notes that nitric acid alone cannot be relied on to remove enough surface metal. SSINA adds that sand and grit blasting should not be used on stainless. The 316L "L" grade matters here too: low carbon protects the heat-affected zone, as explained in 304 vs 316L stainless.

The passive film and passivation

Stainless steel passivates itself: whenever a clean surface meets enough oxygen, a chromium-rich oxide layer forms "automatically and instantaneously". Steel delivered from mills and reputable stockholders is already fully passive, and Euro Inox describes acid passivation as "the exception, rather than the rule". It earns its place when fabrication has left free iron or other contamination on the surface: cutting with tools also used on carbon steel, or steel dust from grinding nearby.

The reference documents are ASTM A967, which covers chemical passivation treatments (nitric acid, citric acid and electrochemical) and free-iron testing, and ASTM A380, which covers cleaning, descaling and passivating stainless parts, assemblies and installed systems. Passivation removes no metal; pickling does, which is why heat tint needs pickling rather than passivation alone. If your specification calls for passivation, name the standard and the test you want.

Galvanic contact with other metals

Galvanic corrosion needs three things together: two metals with different corrosion potentials, an electrical connection between them, and an electrolyte, which may be no more than a film of condensation or wet industrial air. Without the moisture film there is no galvanic corrosion, which is why dry interiors are rarely a problem. Stainless is the more noble metal in the usual pairings, so it is the aluminum, zinc or carbon steel next to it that corrodes.

• Fasteners: use stainless fasteners on stainless panels. Euro Inox shows galvanized screws on a stainless cover rusting white within about 12 months in urban air.
• Area ratio: make the small part the noble one. Stainless screws in a large aluminum part are generally fine; a small aluminum or galvanized part pressed against a large stainless surface is the risky combination.
• Wet crevices: permanently wet joints between stainless and aluminum, zinc or galvanized parts are problem areas, and the area-ratio rule does not reliably apply inside them.
• Isolate: where a stainless enclosure mounts to aluminum, galvanized strut or painted steel outdoors, use insulating washers, plastic bushes or pads. Painting only the less noble metal is not a reliable fix.

Mounting details are covered in the enclosure mounting guide.

Drilling and cutting stainless in the field

Austenitic stainless work hardens. If the drill rubs instead of cutting, it glazes a hard layer that the next pass cannot bite. SSINA's machining guidance: keep a positive feed, avoid dwelling and light successive cuts, use lower cutting speeds, and keep tools sharp. IMOA suggests a feed of at least 0.076 mm (0.003 in) per cut so the tool works below the hardened layer. Two more rules protect the surface:

• Use drills, saws, files and abrasives reserved for stainless. IMOA: once abrasive media have been used on carbon steel "they cannot be used on stainless steel".
• Deburr, clean off the swarf and wipe the area; particles of ordinary steel left on stainless cause rust marks.

Every new hole then needs a fitting rated for the enclosure type: see cutouts, cable glands and HMI windows. Better still, send the layout before the enclosure is built and have the openings cut in the shop.

Cleaning and care

BSSA's maintenance guidance starts with a rule of thumb: "clean the metal when it is dirty". For external stainless that may be once to four times a year; in hygienic areas it may be daily. The routine:

1. Wash with soap or mild detergent and warm water, using a sponge, soft cloth or soft-fibre brush.
2. On brushed and polished finishes, wipe along the grain, not across it; a nylon pad is acceptable along the grain.
3. Rinse with clean water and dry.

And what to keep away from the enclosure:

• Steel wool and carbon-steel brushes or scouring pads: the particles they leave rust on the surface.
• Chloride-containing products: BSSA says hydrochloric acid-based cleaners and hypochlorite bleaches "can cause unacceptable surface staining and pitting, and should not be used".
• Concentrated bleach-based disinfectants, which can damage stainless. Where chlorinated sanitizers are part of the plant routine, rinse after use and consider 316L.

Grade choice for coastal sites and sanitizers is covered in 304 vs 316L, and washdown practice in washdown enclosures for food and beverage.

What to put on a stainless enclosure specification

Frequently asked questions

Sources

1. Metal Supermarkets, sheet metal gauge chart (stainless vs carbon steel gauges)
2. Metal Supermarkets, "Sheet metal gauge chart" (gauges are neither standard nor metric)
3. SSINA, Designer handbook: stainless steel for architecture (gauge table; 2B, BA and No. 4 definitions)
4. ATI, Types 302/304/304L/305 datasheet (density)
5. ASSDA, 2B, 2D and BA cold rolled finishes (ASTM A480 / EN 10088 definitions)
6. ASSDA, mill forms (2B and No. 4 roughness, No. 8)
7. ASSDA, "No. 4: the workhorse finish"
8. BSSA, Specifying finishes for stainless steel flat products (typical Ra by finish)
9. BSSA, surface finish and corrosion resistance of polished stainless steel (Ra above 1 µm; "satin polish")
10. Euro Inox / worldstainless, Stainless Steel in the Food & Beverage Industry (EHEDG 0.8 µm Ra; crevices)
11. ASSDA, Code of Practice for the food industry (intermittent welds and lap joints)
12. ASSDA, FAQ 6 (crevices and intermittent welds trap water and salts)
13. SSINA, Pitting and crevice corrosion
14. BSSA, Post-weld cleaning and finishing of stainless steels (heat tint removal)
15. Euro Inox, Pickling and Passivating Stainless Steel (heat tint, self-passivation, acid passivation)
16. SSINA, Fabrication of stainless steel (no sand or grit blasting; pickling)
17. ASTM A967/A967M-25, Chemical Passivation Treatments for Stainless Steel Parts
18. ASTM A380/A380M-25, Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems
19. Euro Inox, Stainless Steel in Contact with Other Metallic Materials (galvanic corrosion)
20. SSINA, Stainless steel fasteners (galvanic corrosion, more noble metal for the smaller part)
21. SSINA, Machining stainless steels (positive feed, sharp tools)
22. IMOA, Practical Guidelines for the Fabrication of Austenitic Stainless Steels (feed rate, dedicated tools)
23. Euro Inox, "Cleaning Architectural Stainless Steel" (vertical polishing lines outdoors)
24. BSSA, SSAS 7.20 Care and Maintenance of Stainless Steel
25. Euro Inox, cleaning stainless steel (wipe along the grain; no steel wool; bleach)